The invention concerns a support system for a variable-crown roll, wherein the roll comprises a non-revolving roll axle with a rotatable roll mantle displaced on the axle. Between the roll mantle and roll axle are displaced hydraulic loading elements which act upon the underface of the roll mantle in the nip plane. The hydraulic elements are supported on the roll axle, and are loaded by means of a hydraulic pressure medium. The hydraulic elements include pistons fitted into cylinder bores or equivalent formed in the roll axle as well as glide shoes supported on the inner face of the roll mantle.
In the paper machines and equivalent apparatus, such rolls are commonly used to form a smoothing nip, a dewatering press nip, or a calendering nip with a counter-roll. For these purposes, it is important that the distribution of a linear load in the nip, i.e. the profile, in the axial direction of the rolls be made invariable, or that this profile can be regulated as desired. This permits controlling the transverse moisture profile and/or the thickness profile (caliper) of the web of paper or similar material. For this purpose a number of different variable-crown rolls are known in the prior art for controlling the distribution of a linear load in the nip.
In the prior art, a number of different variable-crown rolls for paper machines and equivalent apparatus are known. As a rule, these rolls comprise a massive or tubular, stationary roll axle and a roll mantle disposed for revolving around the axle. Between the axle and the roll mantle, and acting upon the inner face of the mantle, glide shoe arrangements and/or a chamber of pressure fluid or a series of chambers are fitted, so that the axial profile of the mantle at the nip can be aligned or regulated as desired. As a rule, the nips formed by such rolls, such as press nips or calendering nips, are loaded by means of loading forces applied to the axle journals of the variable-crown roll and of its counter-roll.
The present invention relates to such variable-crown rolls which comprise a series of glide shoes whose glide faces act on the inner face of the roll mantle and whose glide faces are at least partly hydrostatically or hydrodynamically lubricated by means of a pressure fluid passed to the glide face.
With respect to the prior art related to the present invention, reference is made, by way of example, to the DE Published Patent Application No. 22 29 421 and to the FI Patent No. 59,655. In the DE publication, the glide shoe arrangement is described that is used perhaps most commonly in variable-crown rolls. In the structures in accordance with the DE publication, the hydraulic loading elements by means of which the roll mantle is supported on the non-revolving roll axle, includes pistons fitted for displacement in cylinder bores provided in the axle, there being the guide shoes which act on the inner face of the roll mantle, and are supported on the pistons by means of articulated joints. In such an arrangement, very high forces are obtained in the articulated joint between the glide shoe and the piston, for which reason the articulated joint must be hydraulically relieved in order that its service life can be made sufficiently long. In spite of the relief, a friction of a certain magnitude is always present in such an articulated joint, as a result of which the glide shoe cannot be fully freely positioned in relation to the piston. Owing to the articulated joint, the sealing arrangements of the loading member may also become quite problematic. Owing to all this, the cost of manufacture of such a loading element provided with an articulated joint is high, and the operation of the element is not satisfactory in all circumstances because of the friction present in the articulated joint.
In the cited FI Patent, the construction of the hydraulic loading member is accomplished such that the loading element is formed as a rigid piece which has no articulated joint. In the FI patent, this is carried into effect in two alternative ways, according to one embodiment of which cylinder bores have been formed in the roll axle, the piston parts of the loading members being fitted for movement in the cylinder bores. In this embodiment, the piston parts have been shaped to be spherical so that they can turn in the cylinder bores. In the other embodiment, pin-like projections corresponding to the cylinder bores have been forced on the roll axle, the glide shoes being supported and sealed pivotally on the projections. In the first embodiment of the cited FI patent, the support point of the glide shoe is placed quite far from the inner face of the roll mantle, whereby the construction is quite unstable. On the other hand, in the second embodiment, the glide shoes must be able to move a relatively long distance in the radial direction of the roll. Both of these embodiments are quite problematic in view of the sealing and, moreover, the positioning of a slide shoe against the inner face of the roll mantle is hampered by friction at the sealing point. The shoe must be able to move by a distance equalliug the stroke length of the loading element.